I had an interesting conversation about 4e yesterday that revealed something I've been waiting for. Somewhere along the way, it has crossed the tipping point where the way to do really awesome things with it is not by adding things, but rather by taking them away.
This point was easy to see coming. As early as the PHB2 it was reasonable to look at things and think "What kind of setting would I have if I removed this class or power source?" This kind of pruning makes for a great thought exercise, but early on it had the problem that if you removed any significant portion of the game, you were limiting the range of available play. If, for example, you were to remove all Arcane classes back when the only options were PHB1 and PHB2, you've just really diminished your options.
But now there is enough material that a decision like that is a lot less impactful. Yes, you might create a problem for someone who wants to play a specific class because they want that specific class, but you're not creating a situation where a player has a really narrow class selection if they want to play a particular style. The bucket is big enough that you can take a big scoop out and still have a large element remaining.
One very nice example of this was put forward by Gamefiend on twitter, suggesting that you could treat the new Thundercats as all being psionic heroes (an idea I like at least in part because none of them have the stupid looking halos that apparently denote psionics in default 4e). Story-wise this is pretty cool: they have a world where powers are defined by certain boundaries (the psionic power source) but are now encountering enemies and ancient mysteries outside that understanding (the arcane power source). It's a classic theme, and it's classic because it works well.
Now, admittedly 4e supports this sort of things very haphazardly. Power sources have very little mechanical weight, and they have almost no meaning beyond how they apply to character classes - settings, monsters and the like have no real resonance with these ideas, which is kind of a shame.
However, while the idea has very little support, it's very supportable (and one could point out that the Dark Sun setting is pretty good evidence of this). The rub is it's never going to be an idea that WOTC is really going to get behind because it hinges on removing things, and that's bad for their business model. But they've made it pretty easy for a DM to decide what power sources mean in his world and remove things that suit his sensibilities.
Now, obviously, there's more to this than the DM tossing things willy-nilly, but I wan tot come back to the premise: 4e has reached the point where you'll get more out of it by treating design of your game as sculpture rather than painting - what you add is less important than what you take away.
(Huh. Note to self - maybe the alternative to multi-classing rules is multi-power-sourcing rules. What happens when your Warlock switches from Arcane to Divine? Must think on this. )